This invention relates to vibration damping in high speed turbomachinery such as hot gas turbine engines, for example, aircraft gas turbine engines, and more particularly to an improved piston ring seal for squeeze film damping means associated with bearing supports for the shaft and turbine rotor of aircraft gas turbine engines.
The occurrence of cyclical orbital motion of the turbine wheel shaft of a hot gas turbine engine is well known in the prior art and hydraulic damping means, for example, squeeze film dampers are utilized to damp vibrations caused by an imbalance in the rotary mass with resultant orbital movement of the turbine wheel shaft. In general such dampers include the use of an oil under pressure in a defined annular squeeze film space between a bearing support member such as the annular outer race of a rolling element bearing and an opposing cylindrical wall of a chamber in the bearing supporting housing in which the race is confined with permitted limited radial and orbital motion.
In such devices as described, circular metal piston seal rings are often employed to control leakage or passage of damper oil out of the defined annular squeeze film space. The rings may be disposed in grooves in the outer race. The pressure of the damper oil being sealed is typically used to urge the piston ring into lateral sealing engagement with a groove sidewall, and into radial sealing engagement with the opposing cylindrical wall of the chamber.
Such rings are usually formed from a length of metal strip into a circular configuration to have closely spaced abutting ends defining the end gaps of a piston ring seal. These end gaps permit installation of the piston ring in the damper, allow for dimensional variations due to thermal growth and mechanical tolerances, and permit the ring to expand and contract during varying damper operating conditions to maintain radial peripheral sealing engagement with the opposing cylindrical wall of the chamber. The end gaps can be sized to permit a small, regulated amount of flow past the piston ring seal, but are normally kept small, and may include overlapping ends to reduce leakage.
Lateral and radial sealing of the ring is a very important factor of damper operation. As mentioned above, known designs use damper fluid pressure to urge the ring laterally into sealing engagement with a groove sidewall and radially into sealing engagement with the opposing cylindrical wall of the chamber.
However, use of the damper fluid pressure for ring sealing can be disadvantageous for a number of reasons. For example, while piston rings in the typical automotive applications are subject to circumferentially uniform pressures, piston rings in squeeze film dampers are subject to a circumferentially varying pressure wave in the damper squeeze film, which will be described in greater detail below. The pressure wave has circumferentially traveling high and low pressure regions. The high pressure region of the pressure wave may act to radially unseat the piston ring from radial engagement with the opposing cylindrical wall of the chamber, with resulting leakage and reduced damper effectiveness. Even where the groove and ring are carefully designed for proper radial pressure balance on the piston ring, unseating and leakage may occur under some operating conditions.
The low pressure region of the traveling pressure wave, which may be lower than the ambient or sump pressure, may act to laterally unseat the piston ring from lateral engagement with a groove sidewall, resulting in leakage, as well as air entrainment in the squeeze film space.
In addition, lateral sealing of the piston ring against a groove sidewall generates friction forces which resist radial movement of the piston ring in its groove. These friction forces diminish the rings ability to maintain radial sealing engagement with the opposing cylindrical wall as pressure in the squeeze film varies due to the traveling pressure wave.
Because of these difficulties in sealing brought on by the variations in damper fluid pressure, scientists and engineers continue to search for improved means of sealing in squeeze film dampers.